The invention is in the field of glare-protection devices, which, for example, are utilized as viewing windows for protection masks, helmets or goggles for welders. The invention is also related to the field of electro-magnetic compatibility (EMC) and concerns an EMC screening element for use in a glare-protection device.
Modern glare-protection devices, which are utilized, for example, as viewing windows for protection masks,xe2x80x94helmets andxe2x80x94goggles for welders, as active filtering elements typically contain at least one liquid crystal (LC) cell, which blocks the light transmission to a greater or lesser extent, as soon as the external light intensity exceeds a predefined threshold. For the detection of the light intensity a light sensor is utilized. An electronic circuit in the glare-protection device comprises an evaluation circuit for the sensor signal and a driving circuit for the liquid crystal cell.
The light sensor and the evaluation circuit detect so-called flickering light in the welding arc. Since modern welding processes operate with very low current values, the signal to be detected has an extremely low signal to noise ratio. For this reason it is difficult to differentiate between the flickering light content and other light contents and to extract the signal to be detected from the sensor output. In order for the evaluation circuit to be capable of doing this, it has to be designed to be very sensitive, i.e., with a high amplification. Such sensitive evaluation circuits, however, have the disadvantage that they are also sensitive to electro-magnetic interference signals. Interfering electro-magnetic influences can, for example, be produced by radiation from electric motors, from power inverters (e.g., of welding installations), from mobile telephones, etc.
Efforts up until now to find a solution to this problem, on the one hand, applied themselves to the evaluation circuit itself. It was attempted to design the evaluation circuit such that it picks up as few interfering signals as possible, and wherein, for example, critical conductor tracks are designed to be as short as possible.
On the other hand, one has recognized that the evaluation circuit also should be screened towards the outside against electro-magnetic interference. In doing so, one up to now concentrated on the outside or front side of the glare-protection device. In this regard, the outside or front side of the glare protection device is the side that is facing away from the carrier person and facing in the direction from which electro-magnetic interference signals are primarily expected. A common measure against such interference signals consists in equipping with electronic components only that side of the circuit board, which is facing inwards and to leave the surface facing outwards free of any components; simultaneously the external surface of the circuit board without any components is equipped with electro-magnetic screening means. Such screening means typically are a fine-mesh screen made of metallic conductor tracks. While these measures do produce some improvements, it has, however, become clear that the EMC screening achieved in this manner is still unsatisfactory for particularly sensitive circuits.
It is an objective of the invention to screen the evaluation circuit or parts of it from interfering electro-magnetic influences or interfering radiation in a better way than by the measures common up until now.
In accordance with the invention, electro-magnetic screening of the electronic components of the evaluation circuit takes place not only on the side of the printed circuit board without any components (i.e., from outside), but also on the side of the circuit board containing components (i.e., from inside). Foreseen therefore is an additional xe2x80x9cprotection behind the frontxe2x80x9d. This measure in accordance with the invention leads to essential improvements in comparison with the up to the present moment usual protection xe2x80x9cat the frontxe2x80x9d.
The glare-protection device in accordance with the invention contains an active filtering element with a light transmission from an external half-space into an internal half-space, which can be influenced, and electronic components for the influencing of the filtering element, which are attached to at least one surface of a circuit board. The glare-protection device furthermore comprises a screening element containing electrically conductive material for the screening of electronic components against electro-magnetic radiation, which screening element is affixed to the at least one surface of the printed circuit board.
The screening elements in accordance with the invention for utilization in the glare-protection device contain electrically conductive material and has a concave shape.
The invention makes it possible to design the evaluation circuit to be exceedingly sensitive, without it being excessively subjected to interference by electro-magnetic influences. Interfering electro-magnetic influences on the one hand, as described above, emanate from the surroundings of the glare-protection device and are referred to hereinafter as external interaction. On the other hand, it has become manifest, that interfering electro-magnetic influences can also be produced within the glare-protection device itself, for instance in the driving circuit for the LC -cell. This is because glare-protection devices of today utilize digital modules, which generate interfering electro-magnetic radiation and also emit these to their surroundings. Also the LC cell itself can exercise undesirable electro-magnetic influences on the evaluation circuit. This xe2x80x9cinternal interactionxe2x80x9d is particularly a problem when the glare-protection device has to be implemented in a small space. The concept in accordance with the invention (i.e., protection behind the front) makes possible a protection also against the internal interaction. This at first unexpected advantage is obviously jointly responsible for the exceptional effectiveness of the invention.
In many instances, the invention even makes possible a simplification of the evaluation circuit, because thanks to the EMCxe2x80x94screening one can make do without certain electronic filter elements. A further benefit of the invention consists in the fact that the screening protects the circuit not only against electro-magnetic radiation, but also against other detrimental physical and/or chemical influences, for example, against perspiration vapors from the carrier person.